Pressurized-water nuclear reactor vessels consist of a shell of overall cylindrical shape, closed at its ends by dished bottoms. The vessel containing the core is exposed to intense neutron radiation and needs to withstand the temperature and the pressure of the primary fluid, consisting of water at a temperature in the region of 320.degree. C. and at a pressure of the order of 150.times.10.sup.5 Pa. The vessels of pressurized-water nuclear reactors which are at present in use have a thickness which is generally between 200 and 300 mm and must to be constructed from cylindrical shells and from a first dished bottom, these components being assembled by welding. The second dished bottom of the vessel forms the closure head which is fastened by means of studs when the vessel is commissioned, onto a flange which is welded onto one of the shells forming the vessel.
The assembly of the vessel is carried out by submerged arc welding with filler metal, and this operation requires the ends of the components to be assembled to be suitably machined in order to delimit two welding chamfers which are then filled with filler metal.
The weld thus produced must then be remachined on the inside and outside and subjected to a heat treatment.
Despite all the precautions taken during the welding operations, the filler metal introduces a certain discontinuity into the metallurgical structure of the vessel metal, and this may be awkward, especially if a welded joint is situated in a region of the vessel opposite the core.
However, it has been possible to obtain a very high degree of safety of the vessels by taking very great precautions during the welding operations and by carrying out many inspections.
The assembly of a nuclear reactor vessel is therefore a very lengthy operation, which requires the use of complex means and the presence of highly qualified personnel. The execution of the filling of the weld chamfers requires many welding passes and the metal which is deposited must to be monitored in order to prevent the presence of any foreign particle in the welded joint (slag inclusions, etc.).
There are known assembly processes using welding without filler metal, which permit a welded joint to be produced in a single pass and over a relatively great thickness.
In particular, welding by means of an electron beam has already been employed for assembling components of relatively great thicknesses.
However, tests have shown that the steels which are currently employed for the construction of light-water nuclear reactor vessels do not permit the welding of components of a thickness greater than 100 mm by means of an electron beam and in a single pass, with a perfect quality of the welded joint.